SINTERED SELF-REINFORCED SILICON NITRIDE

Abstract: A dense, self-reinforced Si3N4 ceramic body and a method of making such body. The body has a composition that includes: (a) Si3N4; (b) a glassy grain boundary phase containing oxygen, nitrogen, Mg, Y, Si, Zr, and at least one of Ti and Al; and (c) a second crystalline phase containing ZrO2; and (d) crystalline phases of metal zirconium silicide and/or metal zirconium silicon nitride. The ceramic exhibits high fracture toughness and high fracture strength and has a density at least 98 % of theoretical.

Description: SINTERED SELF-REINFORCED SILICON NITRIDE TECHNICAL FIELD This invention pertains to a sintered self-reinforced silicon nitride (Si 3 N 4 ) ceramic body and to compositions used in preparing the same. BACKGROUND OF THE INVENTION Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance, and high electrical resistivity. In addition, Si 3 N 4 ceramics resist chemical attack, particularly oxidation. Because of these attributes, Si 3 N 4 is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines. Typically, densification of Si 3 N 4 requires the use of densification aids, such as oxides of magnesium (Mg), yttrium (Y), aluminum (Al), cerium (Ce), silicon (Si), and zirconium (Zr). A powder mixture comprising Si 3 N 4 and one or more of such densification aids is usually prepared and heated under conditions described hereinafter. The densification aids form a liquid phase into which α-Si 3 N dissolves and from which it precipitates as β-Si 3 4 . The final density of a ceramic body depends largely on the density of the body before heat is applied. This is often referred to as "green density," and the body is referred to as "greenware." The most common method of forming greenware is dry pressing. However, a problem with dry pressing is that it does not favor forming bodies that are large or have a complex shape. In addition, forming greenware by dry pressing typically results in low density parts with non-uniform distributions of porosity. The non-uniformity generally relates to a higher density at the edge of a ceramic part than in the center. Further, binders, used to process the bodies into their near net shapes, usually have to be removed. De-bindering is a long and difficult process that often leads to development of internal cracks within the body. An alternate method of forming ceramic greenware is colloidal processing, such as slip-casting. An advantage of colloidal processing is that large, complex shaped, high density greenware can be produced without the use of binders. A second advantage is that aqueous carrier media may be used, eliminating the need for more expensive or potentially environmentally hazardous media, processing conditions or both. Notwithstanding these advantages, colloidal processing has one major disadvantage in that as the number of components in a formulation increases, it becomes more difficult to find common colloidal processing conditions. This is because every component has different surface characteristics, and these characteristics determine the conditions under which colloidal processing will work. Thus, for many multi-component ceramic compositions, it is difficult to form ceramic greenware by colloidal processing. Typically, in order to obtain substantially full densification of the aforementioned powder mixture or greenware, one of four general methods is used: hot pressing (HP), hot isostatic pressing (HIP), pressureless sintering, or low pressure gas sintering. Densification of Si 3 N alone normally does not go to completion in the absence of high pressure. For example, the density of a Si 3 N4 ceramic body might only reach 80 or 90% of its theoretical value. A density of at least 98% of theoretical is required to achieve a ceramic having excellent mechanical and physical properties, such as high fracture strength and high fracture toughness. Further, at high temperatures and low pressures, Si 3 N 4 decomposes into elemental Si and nitrogen. Thus, commercial needs for fully densified Si 3 N4 ceramics, having excellent fracture strength and fracture toughness, are typically met by using HP or HIP to densify combinations of Si 3 N 4 and densification aids. The use of HP or HIP to densify combinations of Si 3 N 4 and densification aids poses two problems. First, both methods require complicated high pressure equipment. Second, the methods typically yield a ceramic body having only a simple shape. In order to obtain a more complicated net or near net shape, the ceramic body that is densified HP or HIP is typically subjected to post-densification procedures such as diamond grinding. Although more complicated shapes may be obtained by pressureless or low pressure gas sintering, these methods typically present difficulties in obtaining substantially full dense ceramic bodies that have high fracture strength and toughness. SUMMARY OF THE INVENTION In one aspect, this invention is a process for preparing a dense, self-reinforced Si 3 N4 (SRS)ceramic body by pressureless or low pressure gas sintering. The process comprises subjecting a powder mixture including: (a) Si 3 N 4 in an amount sufficient to provide a ceramic body; (b) a combination of a source of magnesium oxide (MgO) and a source of silicon dioxide (Si0 2 ), the Si0 2 being present in an amount sufficient to provide a total Si0 2 content of at least 3 weight percent (wt%), the combination being present in an amount sufficient to aid densification of the powder; (c) a source of yttrium oxide (Y 2 0 3 )in an amount sufficient to promote essentially complete conversion of the Si 3 N4to β-Si3N 4 ; (d) a catalytic a mount of a source of zirconium oxide (Zr0 2 ); and (e) at least one whisker growth enhancing compound in an amount sufficient to promote the formation of β-SJ3N 4 whiskers, said compound(s) being selected from a source of titanium dioxide (Ti0 2 ) and a source of aluminum oxide (Al 2 0 3 ); to a pressure within a range of from 1 atmosphere (atm) (0.1 MPa) to 100 atm (10 MPa) and to a temperature sufficient to provide for densification to a value which is at least 98% of theoretical. In addition, the conditions of temperature and pressure are sufficient to provide in situ formation of β-Si 3 N4 whiskers having a high average aspect ratio. In this manner, a dense, SRS ceramic body having a fracture toughness greater than 6.0 MPa-(m)*, as measured by the Chevron notch technique at 23°C, is formed. The process of this invention provides a fully densified, or nearly fully densified, SRS ceramic body by pressureless or low pressure sintering. As used herein, "nearly fully densified" means that the sintered body has a density of at least 98% of theoretical. As used herein, "fully densified" means that the sintered body has a density of at least 99.5% of theoretical. The process of this invention also reduces the need for high pressure equipment, eliminates the need for diamond grinding, and readily provides complicated near net shapes in large numbers. In a second aspect, the process of this invention further comprises a step of forming the powder mixture into greenware prior to sintering. Under this step, the greenware may be formed out of a colloidal suspension comprising the powder mixture dispersed in an aqueous carrier medium, wherein each component of the powder mixture is water dispersible and nonflocculating. In a third aspect, this invention is a Si3 4 ceramic body having a density of at least 98% of theoretical value and a toughness > 6.0 MPa*(m)V as measured by the Chevron notch technique at 23°C. This S13N4 body comprises: (a) a crystalline phase of β-Si3N 4 of which at least 20 volume percent (vol%), as measured by viewing one plane of the Si3N 4 ceramic body by scanning electron microscopy (SEM), is present as whiskers having an average aspect ratio of at least 2.5; (b) a glassy grain boundary phase, in an amount ranging from 2 to 10 wt%, based on total weight of the composition, comprising oxygen, nitrogen, magnesium (Mg), yttrium (Y), Si, Zr, and at least one metal selected from titanium (Ti) and AI; (c) a second crystalline phase comprising Zr0 2 , in an amount ranging from 0.1 to 3 wt% based on total weight of the composition; and (d) at least one additional crystalline phase in an amount ranging from 0.1 to about 3 wt% based on total weight of the composition, the additional phase(s) comprising metal zirconium suicide and metal zirconium silicon nitride, wherein the metal is selected from Ti and Al. In a fourth aspect, this invention is an article of manufacture formed from the above-identified Si3N 4 ceramic body. Examples of possible articles of manufacture are cutting tools and parts in pumps and engines. Articles of manufacture suitable for use in other wear or high temperature applications or end uses are readily determined by skilled artisans without undue experimentation. DETAILED DESCRIPTION OF THE INVENTION The Si3N starting material used in preparing the ceramic body of this invention can be any Si3N 4 powder, including the crystalline forms of α-Si 3 N and β-Si3N 4 , or noncrystalline amorphous SΪ3N4, or mixtures thereof. However, the preferred Si3N 4 powder has a high purity and a high α/β weight ratio, as disclosed in Pyzik, US-A-5, 120,328, col. 4, lines 13- 37, 8-58. For the purposes of this invention, "high" average aspect ratio means an average aspect ratio of at least 2.5. In addition, the powder may be of any size or surface area (SA) provided the dense, SRS ceramic body of this invention is obtained. Preferably, the particles have an average diameter within a range of from 0.2 μm to 5 μm; more preferably, from 0.2 μm to 1 μm. The powder has a SAthat isdesirably within a range of from 5 m * 7g to 15 m ** /g, as determined by the Brunauer-Emmett-Teller (BET) method of measuring SA, which is described in C. N. Satterfield, Heterogeneous Catalysis in Practice 102-05 (McGraw-Hill Book Company, 1980). The range is preferably from 8 m 2 /g to 12 m 2 /g. Oxygen is present to some extent in the form of Si0 2 , usually as a coating on the surface of Si 3 N4 particles. The amount of Si0 2 varies according to the purity of the starting Si3N powder and its method of manufacture. Typically, the concentration of Si0 2 , introduced naturally through the SΪ3N4 powder, ranges from 1.5 to 4 wt% , based on the total weight of the powder mixture. Preferably, total Si0 2 content (amount added plus that present on the surface of the SΪ3 4) is from 3 wt% to 5 wt%. The total Si0 2 content is more preferably from 3.7 wt% to 4.2 wt%. These amounts of Si0 2 are beneficial and aid in densification of the ceramic composition. Typically, if the Si0 2 content of the Si 3 N 4 is less than 3 wt% , additional Si0 2 is added. The Si3N 4 is present in an amount that is suitably within a range of from 90 to 98 wt% based on the total weight of the powder mixture. The range is desirably from 90 to 98 wt% for low pressure gas sintering, and from 92 to 97 wt% for pressureless sintering. Preferably, the range is from 92 to 97 wt% for low pressure gas sintering, and from 93 to 96 wt% for pressureless sintering. A source of Si0 2 and a source of MgO are suitably employed as densification aids in the process of this invention. Silica and MgO function as densification aids because a resulting Si0 2 and MgO containing glass forms a liquid phase at a temperature within a range of from 1300°Cto 1500°C into which α-Si3N4 dissolves. For purposes of this invention, "source" refers to a starting material that is either a desired component of the ceramic body, or a material that converts to the desired component under process conditions. For example, a source of MgO may be MgO itself or any starting material that converts to MgO under process conditions. A specific process condition may influence selection of a particular source. For example, when using a colloidal suspension process with an aqueous carrier medium, a water dispersible, nonflocculating source of MgO, such as magnesium hydroxide (Mg(OH) 2 ), is preferred. During densification of the powdered mixture formed out of this colloidal suspension, Mg(OH) 2 converts to MgO. A "source" may also consist of more than one starting material, as long as the desired component is obtained under process conditions. Any amount of a source of MgO which, in conjunction with the Si0 2 , produces, by pressureless or low pressure gas sintering, the dense, self-reinforced SΪ3 ceramic body of the invention is acceptable. For example, if the source of MgO is Mg(OH) 2 , each wt% of Mg(OH) 2 added to the composition is equal to 0.69 wt% of MgO. Magnesia is suitably present in an amount within a range of from 0.5 to 3 wt% based on the powder mixture total weight (0.7 to 4.3 wt% Mg(OH).). The amount of MgO is preferably within a range of from 0.7 to 2.5 wt% (1 to 3.6wt% Mg(OH).), more preferably, from 0.9 to 2 wt% (1.3 to 2.9 wt% Mg(OH).). The total amount of-densification aid (Si0 2 plus MgO), including the Si0 2 present on the Si 3 4, is within a range that is desirably from 3.5 to 8 wt% based on the total weight of the powder mixture, preferably from 4.6 to 6.2 wt%. In addition to densification aids, the powder mixture must contain a conversion aid. A source of Y 2 θ3 is a desirable conversion aid. The source of Y 2 θ3 may be either Y 2 θ3 or a starting material that converts to Y 2 θ3 under process conditions. As disclosed in Pyzik, US-A- 5,120,328, col. 5, lines 34-52, the conversion aid promotes the rapid, essentially complete conversion of α-Si3N4to β-Si3N 4 . This conversion is most desirable because β-Si3N 4 , in the form of elongated, single crystal whiskers or grains, is important for obtaining the high fracture toughness and high fracture strength of the Si3N 4 ceramic body of this invention. Al I references hereinafter to Si3N4whiskers, single crystal whiskers and single crystal Si3N 4 whiskers are intended to be synonymous and may be used interchangeably. A source of Y 2 0 3 can be employed in any amount, and in any ratio to the densification aids, provided the amount and ratio are sufficient to cause the essentially complete conversion of the starting Si 3 N 4 to β-Si3N 4 and produce the dense, SRS ceramic body of the invention by pressureless or low pressure gas sintering. The amount of Y 2 θ3 provided by a source of Y 2 0 3 is within a range that is desirably from 1 to 6wt%, preferably from 2 to 4wt%, based on the total weight of the powder mixture. The amount of Y2O3 is desirably sufficient to provide a weight ratio of Y 2 θ3 to the combination of MgO and Si0 2 that is within a range of from 2: 1 to 1 :8. The third component required to be present in the powder mixture is a source of a catalytic amount of Zr0 2 . The source of Zr0 2 may be Zr0 2 or any starting material that converts to Zr0 2 under process conditions. As disclosed in Pyzik, US-A-5,120,328, col. 6, lines 14- 33, Zr0 2 functions in a multi-fold manner and is therefore referred to as a "catalyst" for purposes of this invention. For example, Zr0 2 catalyzes the formation of elongated β-Si 3 4 whiskers, and acts as both a densification aid and as a "getter" for metallic impurities, thereby cleaning the glassy phase. Any catalytic amount of a source of Zr0 2 is acceptable provided it is sufficient to achieve its catalytic functions and provide the dense SRS body of this invention by pressureless or low pressure gas sintering. The amount of Zr0 2 provided by the source of Zr0 2 is within a range that is suitably from 0.1 to 3 wt% , preferably from 0.2 to about 1 wt% , based on the total weight of the powder mixture. Thisamount of Zr0 2 can be added to the powder mixture directly, or can be obtained from the use of Zr0 2 balls during attrition mixing of the other powder components. Under typical attrition mixing process conditions of 250 revolutions per minute (rpm) for about one hour, the amount of Zr0 2 introduced is 0.5 wt%. A densification aid (combination of MgO and Si0 2 ) to Zr0 2 weight ratio may be any that produces the dense, SRS body of this invention. The weight ratio is preferably within a range of from 40: 1 to 1 : 1. If the weight ratio f al Is outside of this range, the f i nal density of the ceramic body istypically reduced. The fourth component is a whisker growth enhancing compound. As disclosed in Pyzik, US-A-5, 120,328, col. 7, lines 5-44, this compound helps to provide a ceramic body of superior fracture toughness and fracture strength. In Pyzik, US-A-4,883,776, the presence of calcium, particularly calcium oxide, was found to provide advantages as a whisker growth enhancing compound when Si 3 N 4 powder compositions were hot-pressed into finished ceramic bodies. Subsequently, in Pyzik, US-A-5, 120,328, calcium oxide, tantalum oxide, hafnium oxide, gallium oxide, and indium oxide, were found to promote growth of β-Si3N4 whiskers during pressureless sintering, or low pressure gas sintering, of Si 3 N4 ceramic bodies. It has now been discovered that compounds other than oxides of calcium, tantalum, hafnium, gallium, and indium may be employed to promote whisker growth. The discovery is thatTi0 2 and AI 2 θ3 also promote growth of β-Si 3 N4 whiskers during pressureless or low pressure gas sintering. These compounds can be used as whisker growth enhancing compounds either by themselves, or in combination as a mixture. Using these whisker growth enhancing compounds, ceramic bodies having a fracture toughness > 6 MPa-(m)*, as measured by the Chevron Notch Test, are obtained. Any amount of whisker growth enhancing compound source(s) is acceptable provided it is sufficient to promote the formation of β-SΪ3N4 whiskers, described hereinbelow, and produce the dense, SRS ceramic body of this invention by pressureless or low pressure gas sintering. The source(s) may be Ti0 2 , AI2O3, a mixture of Ti0 2 and AI2O3, or any starting material that converts to at least one of these compounds under process conditions. The amount is desirably sufficient to provide an amount of whisker growth enhancing compound within a range of from 0.01 to 3 wt%, based on the total weight of the powder mixture. The range is preferably from 0.1 to 1 wt% , more preferably, from 0.15 to 0.5 wt% . It is desirable to use sources of MgO, Si0 2 , Y 2 θ3, Zr0 2 , and Al 2 0 3 and/or Ti0 2 in the form of powders that are pure and sufficiently small in size. As disclosed in Pyzik, US-A- 5,120,328, col. 7, line 55 to col.8, line 2, larger amounts of impurities, as for example in the 0.5 wt% range, are not recommended as they may cause a change in the final ceramic composition and properties. In addition, a small powder particle size is favored, preferably having an average particle size < 5 μm in diameter, more preferably, ≤ 1 μm in diameter. In the process of this invention, the starting Si3N 4 powder is mixed with a combination of the densification aid(s), conversion aid, Zr0 2 catalyst, and whisker growth enhancing compound(s) to obtain a mixture that is used in preparing the densified, tough SRS o ceramic body of this invention. It has now been discovered that each component of the mixture may also be derived from many different sources as long as the sources are converted to the respecti e components under process conditions. For example, one source of an oxide component may be difficult to introduce to an aqueous carrier medium, being nondispersible and causing flocculation. A different source of the same oxide component may, however, be 5 water dispersible and nonflocculating. Thus, it is desirable to determine for each component which source may be introduced in combination with other components to obtain effective dispersion of the components without incurring an undesirable amount of flocculation. For example, Mg(OH) 2 , a source of MgO, is water dispersible and nonflocculating, whereas MgO itself flocculates in water. Although this list is not meant to be exhaustive, other examples may 0 be calcium silicate as a source of Si0 2 , calcium zirconate or silicon zirconate as a source of Zr0 2 , aluminum hydroxide as a source of Al 2 03, and calcium titanate or titanium silicate as sources of Ti0 2 . The introduction of calcium, by means of the above described compositions, is also advantageous, since it introduces an additional glass component into the composition, and is useful as a whisker growth enhancing compound in conjunction with at least one compound 5 selected from Al 2 0 3 and Ti0 2 . The total quantity of components other than Si3 4 depends on desired end use applications for the sintered ceramics prepared from the powder mixture. Those skilled in the art recognize that, although the above identified sources of oxide components are used, some of the products derived from these sources under process conditions may also be non-oxide derivatives of Mg, Si, Y, Zr, and at least one compound of a metal selected from Al and Ti. This is acceptable as long as it results in an adequate oxide content and does not adversely affect properties of the resultant sintered body. Ordinarily, the total quantity of MgO, Si0 2 , Y2O3, Zr0 2 , and whisker growth enhancing compound is < 10 wt% of the total weight of the powder mixture. If a source is used that differs from the oxide component, one skilled in the art can calculate the amount of oxide component that will be derived from the source. The total quantity is preferably within a range of from 3 to 7 wt%. The preparation of a finely-divided powder mixture containing Si3N 4 , and sources of MgO, Si0 2 , Y- . O3, Zr0 2 , and whisker growth enhancing compound(s) is accomplished in any suitable manner with conventional apparatus. Ball or attrition-milling of the components in powder form is an acceptable manner of preparation. If Zr0 2 balls are used as milling media, powdered Zr0 2 may not need to be added to the powder mixture, depending on the amount of Zrθ2 required by the composition, because it is obtained from the Zrθ2 balls. After attrition- -milling, the attritor balls are removed, and any conventional method of forming the composition into pre-sintering greenware may be utilized. Generally, it is advantageous to prepare the above described Si3N4 composition by use of a carrier medium. The powders and carrier medium form a solid suspension having a viscosity suitable for mixing. Preparation of the above described composition in a carrier medium requires no particular order of addition of the components. For example, it is possible to add the powdered combination or one or more individual components thereof to a colloidal suspension of Si 3 N4 in the carrier medium or vice versa. Alternatively, all components of the powder mixture may be added simultaneously to the carrier medium prior to attrition-milling. The process is typically conducted in a large vessel at room temperature (taken as 23°C), under air, with vigorous stirring. This is one method of deagglomeration mixing, but any common stirring means is suitable, such as the above described ball-milling device or attrition mixer. An ultrasonic vibrator may be used in a supplementary manner to break down smaller agglomerates. However, the attrition mixer is preferred. The carrier medium may be any inorganic or organic compound that is a liquid at room temperature and atmospheric pressure and in which the powdered sources of the components can be dispersed. Any quantity of carrier medium that achieves its purpose is sufficient and acceptable. Suitably, the carrier medium is aqueous and is employed in a quantity sufficient to provide a solids content within a range of from 15 to 50 vol%, based upon total volume of carrier medium and solids. Preferably, the range is from 35 to 44vol% . Below the 15 vol% lower limit, the viscosity of the solid suspension may be too low and deagglomeration mixing may be ineffective. Above the 50 vol% upper limit, the viscosity may be too high and deagglomeration mixing may be difficult. To aid in dispersing components of the powder mixture into the carrier medium, one or more surfactants or dispersants may be added to the suspension. The choice of surf ac- tant(s) or dispersant(s) can vary widely as is well-known in the art. Any surfactant or dispersant is acceptable if it improves dispersion of the powder mixture components and does not adversely affect other process conditions or characteristics of the resultant product. Preferably, the surfactant for an aqueous carrier medium is KV 5088 (Zschimmer & Schwarz). With an aqueous-KV 5088 carrier medium, a colloidal suspension can be slip-cast or pressure cast to a green density above 55% of theoretical. In contrast, when using an alcohol carrier medium such as methanol, the KV 5088 dispersant is not advantageously used. Instead, a fiocculant such as oleic acid may be used to ease recovery of the powder mixture. In this case, colloidal processing, such as slip-casting, is usually not used because the density of the resulting greenware is believed to be too low. When an alcohol carrier medium is used, the powder is preferably dried, sieved, and dry-pressed into greenware. Any amount of surfactant or dispersant is acceptable provided dispersion of the powder mixture components is improved. The amount of surfactant is typically within a range 5 of from 0.01 to 1.5 wt%, based on powder mixture weight. When using the preferred aqueous carrier medium, the amount of KV 5088 surfactant is preferably within a range of from 0.5 to 1.5 wt%, more preferably from 0.7 to 1.2 wt%, based on powder mixture weight. Once the Si 3 N4, and sources of MgO, Si0 2 , Y2O3, Zr0 2 , and whisker growth enhancing compound(s) are finely-divided and dispersed, they may be formed into greenware, k 10 preferably by slip-casting. After slip-casting, the greenware is further dried under standard drying conditions. During the drying process, additional free carrier medium is removed. The drying temperature depends on the boiling point of the carrier medium employed. Drying is typically conducted at a temperature below the boiling point of the carrier medium under atmospheric pressure. For example, drying of greenware when an aqueous carrier medium has 15 been employed is conducted by gradually heating the greenware to a temperature of 80°C and remaining at that temperature for 24 hours. For methanol, the same procedure is conducted, but the maximum temperature should be between 60°C and 70°C. A desirable green density obtained from the slip-casting method is greater than 55% of theoretical density. However, this depends upon the vol% of sol ids in the suspension. 20 Any other appropriate method for processing the composition into greenware may also be used. For example, an alternative to slip-casting would be to simply dry the suspension into a powder, grind it, pour it into a mold of the desired shape, and dry press. This process is disclosed in Pyzik, US-A-5, 120,328, col. 9, lines 31-65. Any standard equipment suitable for pressureless or low pressure gas sintering is 25 acceptable provided it yieldsthe dense, SRS ceramic body of this invention. Those skilled in the art will recognize that pressureless and low pressure gas sintering are difficult densification methods and that compositions which can be densified by these methods may also be densified by hot pressing (HP) or hot isostatic pressing (HIP). Standard equipment and pressure conditions for pressureless and low pressure gas sintering are disclosed in Pyzik, US-A-5, 120,328 30 at col. 10, lines 1-19. Typically, the greenware is placed into a graphite or boron nitride crucible containing a Si3N 4 powder bed, or more preferably, a powder bed of Si3N 4 and oxides of the same metal components present in the greenware (MgO, Si0 2 , Y2O3, Zr0 2 and at least one of either Ti0 2 or AI2O3). The pressure typically ranges from 1 atm (0.1 MPa) to 100 atm (10 MPa). Any sintering temperature and amount of time at the sintering temperature will 35 suffice provided the dense, SRS ceramic body of this invention is formed. For purposes of this invention, suitable sintering temperatures and amounts of time at these temperatures are disclosed in Pyzik, US-A-5, 120,328 at col. 10, lines 20-56. The sintering temperatures preferably range from 1650°C to 1825°C, more preferably, from 1700°Cto 1750°C. Typically, it takes from 2 hours to 3 hours to heat the sample up to the sintering temperature. At the desired sintering temperature, the sample is preferably sintered for a period of from 3 to 36 hours, more preferably from 12 to 15 hours. Afterwards, the sample is cooled to room temperature over a period of 2 hours. Some variation in the preferred temperature range may be observed depending on the method employed in measuring the temperature. The above sintering temperatures are measured by use of a tungsten-rhenium thermocouple obtained from, and calibrated by, the Omega Company. The pressureless sintering and low pressure gas sintering methods described hereinbefore allow for the formation of various Si 3 N 4 ceramic articles of manufacture such as o cutting tools or parts in pumps and engines. A variety of shapes, such as flat plates or tubes, can be prepared. In addition, using a colloidal suspension, articles of manufacture can be fabricated by slip-casting into greenware having a variety of shapes. The subsequent pressureless and low pressure sintering methods of this invention typically do not require any slicing or grinding of the slip cast to obtain the substantially fully dense near-net shape. 5 The SRS ceramic body produced by the process of this invention is a dense material having no significant porosity. Preferably, densification proceeds to a density of greater than 98% of theoretical; more preferably, to greater than 99% of theoretical; and most preferably, to greater than 99.5% of theoretical. Those skilled in the art will recognize that the theoretical density will vary depending upon the composition of the ceramic. 0 As measured by X-ray diffraction (XRD), the Si3N 4 in the densified ceramic body is present in the beta (β) crystalline form, indicating essentially complete alpha (α) to β conversion during processing. The β-Si3N is present predominately as single crystal, "needle-like" whiskers or elongated grains, as determined by both SEM and transmission electron microscopy (TEM). The size of the β-Si 3 N4 grains is usually in a range of from 1 μm to 20 μm in length with a 5 mean diameter of from 0.2 μm to 5 μm; preferably, with a mean diameter of from 0.3 μm to 1 μm. As disclosed in Pyzik, US-A-5, 120,328 at col. 1 1, lines 29-54, and measured by SEM, the whiskers are oriented randomly, and the volume occupied by the whiskers is approximately the same in all planes throughout the ceramic body. The percentage of Si 3 N4 whiskers having an aspect ratio of between 2 and 16 is desirably at least 20 vol%, preferably at least 35 vol%, as measured in a plane. The average aspect ratio of the Si 3 N 4 whiskers is desirably at least 2.5, preferably at least 4. In addition to a Si3N 4 crystalline phase, the body of this invention contains a glassy grain boundary phase in an amount ranging from 2 to 10 wt%, preferably from 5 to 7 wt%, based on total bodyweight. The glassy phase comprises oxygen, nitrogen, Mg, Y, Si, Zr, and at least one metal selected from Ti and Al. In the glassy phase, the elements may be bonded in any number of different ways including: metal oxides, metal nitrides, metal oxynitrides, mixed metal oxides, mixed metal nitrides, and mixed metal oxynitrides, wherein the metals are selected from Mg, Y, Si, Zr, Ti, and Al. Additionally, the body of this invention contains a second crystalline phase in an amount suitably ranging from 0.1 to 5 wt% of total body weight. The second crystalline phase is Zr0 2 . Two other crystalline phases may also be present an amount totaling not more than 2 wt%. Specifically, these other crystalline phases comprise metal zirconium suicide and metal zirconium silicon nitride. The metal is at least one of Ti orAI. The mechanical properties of the SRS ceramic body of this invention are readily measured by use of standard tests. In particular, fracture toughness (K iC ) is measured according to the Chevron notch and the Palmqvist methods. Fracture strength (modulus of rupture) is measured according to the Military Standard 1942b test. Hardness is measured according to the Vickers indentation test. These standard tests are thoroughly discussed in Pyzik, US-A- 5,120,328 at col. 12, line 64 to col. 14, line 24. Fracture strength (modulus of rupture) measures the resistance of the material to fracture under a load. Typically, the fracture strength at room temperature is at least 95 ksi (650 MPa). Preferably, the fracture strength at room temperature ranges from 1 16 ksi (800 MPa) to 130 ksi (900 MPa). Typically, at 1200°C, the fracture strength is at least 65 ksi (450 MPa). Toughness measures the resistance of the material to fracture under a dynamic load. Typically, the fracture toughness of the densified SRS ceramic body of this invention, as measured at room temperature by the Chevron notch technique, is > 6 MPa-fm)*. The room temperature fracture toughness ispreferably > 7 MPa*(m)÷; more preferably, > 7.5 MPa-(m)*; and most preferably, > 7.8 MPa*(m)÷. The Vickers hardness test measures the resistance of a ceramic material to indentation. The room temperature Vickers hardness of the SJ3N4 ceramic bodies of this invention is at least 1400 kg/mm 2 , preferably from 1425 to 1600kg/mm 2 and more preferably from 1450 to 1600 kg/mm 2 . The Palmqvist toughness test is an extension of the Vickers test. Preferably, the Si3N 4 ceramic bodies of this invention exhibit a Palmqvist toughness at room temperature of at least 37 kg/mm, more preferably from 37 to 52 kg/mm, and most preferably from 40 to 46 kg/mm. Illustrative Embodiments The following examples serve to illustrate the novel, dense, self-reinforced SΪ3N 4 bodies of this invention and the method of preparing said bodies. The examples are not intended to limit the scope of this invention. The Si 3 N 4 powder was commercially available from Ube Industries, Ltd. under the trade designation SN-E10. It contained: 1.6 % oxygen; less than 100 parts per million parts of powder (ppm) CI ; less than 100 ppm Fe; less than 50 ppm Ca; and less than 50 ppm Al . It had a crystallinity of greater than 99.5%, a ratio of β/(α + β) of less than 5, and a SA of 1 1.2 m 2 /g. Moly Corp supplied Y 2 0 3 , and Baker Incorporated supplied Mg(OH) 2 . Zr0 2 , in the form of Zr0 2 balls, was available from Union Process. Zr0 2 powder was available from Toyo Soda (type TZ-3Y20A). Ti0 2 and Al 2 0 3 powders were available from Alfa Products. Example 1 (a) Colloidal Processing and Pressureless Sintering A series of SJ3N 4 greenware was prepared according to a general method that began by mixing desired amounts of Si3N4, Mg(OH) , Si0 2 , Y2O3, and a whisker growth enhancing compound (Ti0 2 , AI θ3 or a combination thereof) in an attritor (Union Process batch attritor, Model 01 HD - 750 cm3 capacity with polytetrafluoroethylene coated tube and stirrer) containing Zr0 2 balls and a carrier medium with a stirring rate of 250 rpm for a mixing time of 1 hourto form a slurry. Deionized (Dl) water with 0.9 wt% KV 5088 surfactant was used as the carrier medium in an amount sufficient to provide a solids content of 35 to 40 wt% solids. The slurry was poured through a 400 (37 μm) mesh plastic sieve to remove the Zr0 2 balls. The slurry was then poured into a slip-casting form positioned on gypsum plaster. The cast was dried at room temperature for 24 hours and then dried under a flow of dry nitrogen gas at a temperature of 80°C for 24 hours to form dried greenware. Greenware having a size of 2 inches (5.1 cm) in diameter by 1.5 inches (3.8 cm) in width, and 0.5 inch (1.3 cm) in thickness was placed in a graphite crucible filled with a Si3 4 powder bed containing oxides of the same metal components present in the greenware (MgO, Si0 2 , Y2O3, Zrθ2, and at least one of either Ti0 2 or AI-JO3). The greenware was then pressureless sintered for 12 hours in a 4 inch (10.2 cm) ASTRO™ graphite furnace at 1750°C under a nitrogen atmosphere of 1 atm (0.1 MPa) pressure. Samples prepared by this method are set forth as Samples (a)-(e) in Table I. (b) Dry Pressing and Pressureless Sintering Powdered mixtures of SJ3N4, MgO, Si0 2 , Y 2 θ3 and at least one whisker growth enhancing compound (Ti0 2 or AI-JO3) were prepared by addition to a methanol slurry and attrition milling. After attrition milling, instead of slip-casting, the samples were dried for 24 hours at 70°C to provide dried powder mixtures. A sample quantity (80 g) of each dried powder mixture was poured into a stainless steel die 2 inches (5.1 cm) in diameter by 0.5 inch (1.3 cm) in thickness. The sample quantity was dry pressed at room temperature under a pressure of 5 psi (34.5 kPa) and then isostatically pressed at room temperature under a pressure of 45 ksi (310.3 MPa). The resulting pressed disc was placed in a graphite crucible, and the crucible is filled with a SΪ3N4 powder bed containing oxides of the same metal components present in the greenware. The disc was thereafter sintered as described in Example 1(a), above. Samples prepared by this method are set forth in Table I as Samples (f)-(i). The densities of the Si3N 4 ceramic bodies, prepared hereinabove, were measured by the water immersion method, as described in "Modern Ceramic Engineering" by D. W. Richerson, Marcel Dekker, 1982, and by stereology analysis from SEM photomicrographs with the results shown in Table I. The greenware compositions of Table 1 were pressureless sintered to form ceramic bodies having densities of at least 98% of theoretical. 5 10 15 20 25 30 35 Table I* n Sample → I.D. % S i 3 U % Mg(0H) 2 % MgO % Si0 2 % Y 2 °3 % Zr0 % Ti0 2 % Λ1 2 0 3 Green Sintered Density Density Solvent a 92.68 2.28 - 1.50 2.86 0.50 0.18 - 58.7 99-9 Water rπ b 91.-1-1 .3-76 - 1.50 2.62 0.50 0.18 - 57.2 98.1 Water cn c 92.48 2.28 - 1.30 2.86 0.50 0.18 - 58.1 98.3 Water m∑. d 92.68 2.28 - 1.50 2.86 0.50 - 0.18 56.9 99.3 Water Ώ e 92.48 2.28 - 1.30 2.86 0.50 - 0.18 57.2 99.1 Water f 93-53 - 1.59 1.30 2.88 0.50 0.20 - 53-0 99.0 Methanol g 93.35 - 1.59 1.50 2.88 0.50 - 0.18 52.4 98.8 Methanol m h 93-33 - 1.59 1.50 2.88 0.50 0.10 0.10 52.9 99.0 Methanol ro i 93.30 - 1.59 1.50 2.88 0.55 0.18 - 53-2 99.5 Methanol * Percentages of components are based on vιt% in the powder mixture. (c) Analysis of Slip-Cast Method and Dry Press Method Phases and Morphology Samples (a), (d), and (i) attained densities of 99.9%, 99.3%, and 99.5% of theoretical, respectively. Approximately 35 vol% of each Si 3 N 4 composition contained elongated grains of β-Si3N , the grains having an average aspect ratio of 2.5 and an average diameter between 0.5 μm and 1 μm. The bulk chemical composition of Samples (a), (d), and (i) was measured by neutron activation analysis. Each contained 92 wt% crystalline S13N4, 6 wt% glassy grain boundary phase, and 2 wt% other crystalline phases. The glassy grain boundary phase had an average glass composition, as measured by analytical transition electron microscopy (ATEM), as follows: 37 ± 1 1 wt% Y, 7 ± 5 wt% Si, 6 ± 5 wt% Mg, 6 _+_ 3 wt% Zr, 2 +_ 1 wt% Al for (d), 2 _+_ 1 wt% Ti for both (a) and (i), 39 ±_ 10 wt% oxygen, and 3 _+_ 2 wt% nitrogen. Traces of CaO impurity, coming from the MgO, are also detected in the glassy grain boundary phase. A second crystalline phase of Zr0 2 was found in a concentration of 0.1 wt%. In addition, Samples (a) and (i) had two minor crystalline phases comprising zirconium titanium suicide and zirconium titanium silicon nitride in a total concentration of 2 wt%. Sample (d) had two minor crystalline phases comprising zirconium aluminum suicide and zirconium aluminum silicon nitride in a concentration of 2 wt%. At room temperature, fracture toughness, flexural strength and Vickers Hardness were measured for Samples (a), (d), and (i). The results are shown in Table II. TABLE II Fracture Fracture Sample Toughness Toughness Fracture Vickers I.D. (Palmqvist (Chevron Strength Hardness test-kg/mm) Notch test- (MPa) (kg/mm2) MPa-mi) a 46.0 8.5 862 1570 d 44.2 8.3 827 1580 i 43.3 8.0 827 1550 Example 2 A series of ceramic bodies was prepared as in Example 1 (a), with the exception that the powder mixture comprised 93.3 wt% Si 3 N , 2.9 wt% Y 2 0 3 , 2.3 wt% Mg(OH) , 1.3 wt% Si0 2 , 0.2 wt% Ti0 2 , and a concentration of Zr0 2 as shown in Table III. The ceramic bodies had a sintered density of at least 98% of theoretical. The presence of Zr0 in an amount ranging from 0.2 to 2% led to a sintered density of at least 99% of theoretical. In the absence of Zr0 (Sample a*) or in the presence of excess Zr0 2 (Sample f), lower densities were obtained. TABLE III Sample % Theor. I.D. Wt. % Zrθ2 Density a* 0 97.5 b 0.2 99.3 c 0.5 99.5 d 1.0 99.5 e 2.0 99.2 f 5.0 98.4 * not an example of the invention Example 3. Two greenware samples were prepared as in Example 1(a), with the exception that one powder mixture comprised: 92.9 wt% Si3N 4 , 2.3 wt% Mg(OH) 2 , 1.5 wt% Si0 2 , 2.9 wt% Y 2 θ3, and 0.4 wt% calcium titanate (CaTiOs); and the other comprised, 92.8 wt% Si3N , 2.3 wt% Mg(OH) 2 , 1.0 wt% Si0 2 , 2.9 wt% Y 2 0 3 , and 1.0wt% calcium silicate (CaSi0 3 ). The compositions were separately attrition mixed with carrier mediums of 135 ml of D.I. water and 1.75 ml of KV 5088 surfactant, providing solids loadings of 35 vol%. After dispersion, minimal flocculation was observed. It was also observed that replacing either CaTi0 3 or CaSiθ3 with CaO led to flocculation of the powder mixture in the water carrier medium and precluded preparation of greenware via slip casting. The resultant slurries were slip cast and dried as described in Example 1(a). The resulting ceramic greenware casts had green densities of 53.7% and 54.8% of theoretical, respectively. The greenware casts were then pressureless sintered as described in Example 1 (a) to obtain sintered densities of 99.4% and 99.3% of theoretical, respectively. This example shows that, while CaO cannot be used in a water carrier medium, CaTiθ3 and CaSi0 3 introduce both calcium and either Ti or Si, respectively, into the composition and do not flocculate in water carrier mediums. Similar results are expected with other compositions and process variations, all of which are disclosed in this application.

Claims: 1. A process for preparing a dense, self-reinforced silicon nitride ceramic body by pressureless or low pressure gas sintering, the process comprising subjecting a powder mixture including: (a) Si 3 N 4 in an amount sufficient to provide a ceramic body; (b) a combination of a source of MgO and a source of Si0 2 , the Si0 being present in an amount sufficient to provide a total Si0 content of at least 3 wt%, and the combination being present in an amount sufficient to aid densification of the powder; (c) a source of Y 2 θ3 in an amount sufficient to promote essentially complete conversion of the Si3N to β-Si3N ; (d) a catalytic amount of a source of Zr0 2 ; and (e) at least one whisker growth enhancing compound in an amount sufficient to promote the formation of β-Si 3 N4 whiskers, said compound(s) being selected from a source of Ti0 2 and a source of Al 2 0 3 ; to a pressure within a range of from 1 atm (0.1 MPa) to 100 atm (10 MPa) and to a temperature sufficient to provide for densification to a value which is at least 98% of theoretical and sufficient to provide in situ formation of β-Si 3 N4 whiskers having a high average aspect ratio, such that the ceramic has a fracture toughness greater than 6 MPa-fm^as measured by the Chevron notch technique at 23°C. 2. A process as claimed in Claim 1, wherein the amount of Si3N 4 ranges from 90 to 98 wt% based on total weight of the powder mixture. 3. A process as claimed in Claim 1 , wherein the source(s) of Si0 2 are present in an amount sufficient to provide a total Si0 2 content within a range of from 3 to 5 wt% based on total weight of the powder mixture. 4. A process as claimed in Claim 1, wherein the source of MgO is present in an amount sufficient to provide a MgO content within a range of from 0.5 to 3 wt%, based on total weight of the powder mixture. 5. A process as claimed in Claim 1, wherein the amount of the source of Y 2 0 3 is sufficient to provide an Y 2 03 content within a range of from 1 to 6wt%, based on total weight of the powder mixture. 6. Aprocess as claimed in Claim 1, wherein the amount of the source of Zr0 is sufficient to provide a Zr0 2 content within a range of from 0.2 to 3 wt%, based on total weight of the powder mixture. 7. A process as claimed in Claim 1 , wherein thewhisker growth enhancing compound content is within a range of from 0.01 to 3 wt%, based on total weight of the powder mixture. 8. A process as claimed in Claim 1, wherein the amounts of the sources of Y 2 0 3 , MgO, and Si0 2 provide a weight ratio of Y 2 O3 to the combination of MgO and Si0 2 that is within a range of from 2: 1 to 1 :8. 9. A process as claimed in Claim 1 , wherein the amounts of the sources of MgO, Si0 2 , and Zr0 2 provide a weight ratio of the combination of Si0 2 and MgO to Zrθ2 that is within a range of from 40: 1 to 1 : 1. 10. A process as claimed in Claim 1 , further comprising a step of forming the powder mixture into greenware prior to sintering. 1 1. A process as claimed in Claim 10, wherein the greenware is formed out of a colloidal suspension comprising the powder mixture dispersed in an aqueous carrier medium, each component of the powder mixture being water dispersible and nonflocculating. 12. A process as claimed in Claim 11, wherein the source of MgO is Mg(OH)2. 13. A process as claimed in Claim 1 1 , wherein the aqueous carrier medium is admixed with a surfactant. 14. A silicon nitride ceramic body having a density that is at least 98% of theoretical val ue and a toughness greater than 6 MPa-(m)* as measured by the Chevron notch technique at 23°C, comprising: (a) a crystalline phase of β-Si3N 4 of which at least 20 vol%, as measured by viewing one plane of the Si3N 4 ceramic body by SEM, is present as whiskers having an average aspect ratio of at least 2.5; (b) a glassy grain boundary phase, in an amount ranging from 2 to 10 wt%, based on total weight of the composition, comprising oxygen, nitrogen, Mg, Y, Si, Zr, and at least one metal selected from Ti and Al; (c) a second crystalline phase comprising Zr0 2 , in an amount ranging from 0.1 to 3 wt% based on total weight of the composition; and (d) at least one additional crystalline phase in an amount ranging from 0.1 to 3 wt% based on total weight of the composition, the additional phase(s) comprising metal zirconium suicide and metal zirconium silicon nitride, wherein the metal is selected from Ti and Al. 15. A body as claimed in Claim 14, wherein the glassy grain boundary phase contains Mg, Si, Y, Zr, Ti, oxygen, and nitrogen, and wherein two other crystalline phases of titanium zirconium suicide and titanium zirconium silicon nitride are present. 16. A body as claimed in Claim 14, wherein the glassy grain boundary phase contains Mg, Si, Y, Zr, Al, oxygen, and nitrogen, and wherein two other crystalline phases of aluminum zirconium suicide and aluminum zirconium silicon nitride are present. 17. An article of manufacture prepared from the body of Claim 14.

Inventors: PYZIK ALEKSANDER J, ROSSOW HAROLD E, PYZIK, ALEKSANDER, J., ROSSOW, HAROLD, E.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1994
Main classification: C04B35
IPC codes: C04B35